1. Technical Field
The present invention relates to a method for bending a glass sheet, a ring mold for bending a glass sheet and an apparatus using the ring mold. In particular, the present invention relates to a method for bending a glass sheet to be used for a windshield and a rear window of an automobile and so on, a ring mold for bending such a glass sheet, and an apparatus using the ring mold.
2. Background Art
In general, a glass sheet for a windshield, rear window and the like of an automobile is required to be formed in a bent shape in terms of design in automobiles. The windshield and the rear glass are ones that are obtained by bending a flat glass sheet cut in a certain shape (hereinbelow, referred to as xe2x80x9cthe or an unformed glass sheetxe2x80x9d).
Now, a forming ring mold in an apparatus for bending a glass sheet will be explained. The ring mold is bent so as to be substantially matched with the outline of a final shape (product shape) of a bent glass sheet. The unformed glass sheet has periphery portions thereof supported by the ring mold, and the ring mold with the unformed glass sheet supported thereon is carried into a heating furnace. As a result, the unformed glass sheet is heated to a bending temperature, sags downwardly by its own weight to have the periphery portions bent along the ring mold, and is bent into a certain shape.
An example of the ring mold is one disclosed in JP-A-63156025. This ring mold comprises fixed frames for supporting central periphery portions of a glass sheet, and swingable frames provided adjacent to the fixed frames. The ring mold is carried into a heating furnace, supporting lateral periphery portions of the unformed glass sheet by the swingable frames. When the unformed glass sheet is heated to be soften, the swingable frames swing as the unformed glass sheet is becoming bent. Thus, the swingable frames and the fixed frames are combined to provide a framework having a certain bent shape, and the periphery portions of the unformed glass sheet sags along the framework by their own weight. A central region of the glass sheet simply sags downwardly by its own weight since the central region is not supported by the ring mold.
By the way, automobiles having a complicated bent shape have been frequently seen in recent years. For this reason, a windshield, a rear window and the like are required to have a complicated bent shape. For example, glass sheets that have a central region formed in a greatly bent shape have been increasingly demanded.
In order to bend a glass sheet in such a shape, the above-mentioned publication discloses an arrangement wherein a pressing die, which is provided above the framework, is pressed against the glass sheet. It is difficult to increase only the central region in the bend applied to the glass sheet without using the pressing die. In this purpose, the pressing die is required to be used for assisting the bend.
However, the pressing die is required to be additionally provided, making the apparatus complicated. The use of the pressing die increases the external force applied to the glass sheet, contributing to degradation in optical quality of the glass sheet, such as strain.
As a ring mold to be used for forming a glass sheet in a complicated bent shape, there is one that is disclosed in, e.g., JP-A-8183625. This ring mold includes a first forming frame and a second forming frame. The first forming frame supports the periphery portions of an unformed glass sheet, and the unformed glass sheet is bent to be formed into an intermediate glass sheet when the unformed glass sheet is heated to a bending temperature. The second forming frame has such a shape to be matched with a final shape of the glass sheet to be formed. The second forming frame supports the periphery portions of the intermediate glass sheet, and the periphery portions of the intermediate glass sheet sag along the second forming frame, providing the glass sheet with a desired bent shape.
The method to bend a glass sheet by this ring mold includes a first forming process and a second forming process. The first forming process includes the following steps. An unformed glass sheet is put on the first forming frame, the ring mold with the unformed glass sheet put thereon in such a way is carried into a heating furnace, and the unformed glass sheet is heated to a bending temperature. The unformed glass sheet has the periphery portions sagged along the first forming frame, being formed in the intermediate glass sheet. The second forming process includes the following steps subsequent to the first forming process. The intermediate glass sheet which has been formed in the first forming process is transferred onto the second forming frame. The intermediate glass sheet has the periphery portions sagged along the second forming frame, being formed in a bent shape. Thus, a complicated bent shape is provided to the glass sheet.
In the ring mold disclosed in JP-A-8113625, the movable forming frame is movable about the centerline of the ring mold. In the second forming process, the lateral portions of the movable forming frame move so as to rise up with respect to the centerline. By this movement, the glass sheet becomes opt to jump, contributing to degraded precision in the bent shape of the glass sheet or degraded optical quality due to occurrence of strain and so on. Normally, glass sheets for automobiles have a substantially rectangular shape or a trapezoid shape having short sides and long sides. When a glass sheet having such a shape is bent by its own weight, the glass sheet is easy to be bent in a long side direction and difficult to be bent in a short side direction. From this view point, it is difficult to bend a glass sheet in a sufficient way in the short side direction in the conventional method disclosed in the two publications.
A proposal has been made to increase heating to portions of a glass sheet in the short side direction in order to promote the bend in the short side direction. However, a temperature distribution is required to be finely controlled in order to heat an unformed glass sheet. The fine control of the temperature distribution needs many special heaters, creating problems in that the control is complicated and the cost increases.
It is an object of the present invention to solve the problems of the prior art stated earlier to provide a method for bending a glass sheet, a ring mold for bending a glass sheet and an apparatus using the ring mold, which have not been known.
In order to solve the problems, the present invention provides a method for bending a glass sheet, which comprises putting an unformed glass sheet on a ring mold to support peripheral portions of the unformed glass sheet thereon, and carrying the ring mold into a heating furnace to bend the glass sheet; and which further comprises a first forming process to heat the unformed glass to a bending temperature to bend the unformed glass sheet so as to form the unformed glass sheet into an intermediate glass sheet, and a second forming process to support the peripheral portions of the intermediate glass on a forming surface conformed with a final shape of the glass sheet to be formed and to form the intermediate glass sheet into a glass sheet having a certain bent shape along the forming surfaces; characterized in that the intermediate glass sheet is formed by bending portions of the unformed glass sheet in a short side direction in the first forming process, and the portions of the intermediate glass sheet in the short direction are further bent into a certain bent shape and portions of the intermediate glass sheet in a long side direction are bent.
The present invention also provides a ring mold for bending a glass sheet, which supports peripheral portions of an unformed glass sheet, and is carried into a heating furnace, carrying the unformed glass sheet thereon, to bend the glass sheet, and which comprises a first forming frame for bending the unformed glass sheet to form the unformed glass sheet into an intermediate glass sheet when the unformed glass sheet is heated to a bending temperature, and a second forming frame for supporting peripheral portions of the intermediate glass formed by the first forming frame on a forming surface conformed with a final shape of the glass sheet and for forming the intermediate glass sheet into the final glass sheet along the forming surface, characterized in that the second forming frame comprises fixed frames fixedly provided, and swingable frames provided on both sides of the fixed frames in a longitudinal direction and apart from the fixed frames, and that each of the swingable frames is turnable about an intermediate position as a swingable center which is located apart from a separating position with the related fixed frame.
In addition, the present invention provides an apparatus for bending a glass sheet, which comprises a heating furnace for heating an unformed glass sheet to a bending temperature, and the ring mold for bending a glass sheet to be carried into the heating furnace, supporting peripheral portions of the unformed glass sheet thereon and to sag the unformed glass sheet due to its own weight caused by softening of the glass sheet.